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On the geocentric micrometeor velocity distribution

Diego Janches,1,2 Michael C. Nolan,1 David D. Meisel,1,3 John D. Mathews,2

Qihou H. Zhou,4 and Danielle E. Moser5

Received 21 November 2002; revised 19 February 2003; accepted 3 March 2003; published 4 June 2003.

[1] We report micrometeor velocity distributions measured with unprecedented velocityand radiant resolution using the dual-beam 430 MHz Arecibo (AO) radar in Puerto Rico.The AO radar detects over 10,000 events daily inside its 300 m radar beam that are

produced mainly by particles in the size range 0.5100 microns. During the observationsreported here, the line feed antenna is pointed vertically while the Gregorian feed is

pointed at an angle of 15 degrees from zenith. The off-vertical radar beam is initiallyplaced pointing north and every 30 min is rotated 180 degrees, allowing observation ofthree different regions of the Earths ionosphere. Results from the observations performedon 21 January and 25 June 2002 are presented and discussed. We observe that the

meteoroid population detected by AO is smaller in particle size and faster in velocity andthus entirely different from the one observed by conventional lower-frequency meteorradars that use a different scattering mechanism. We observe qualitative difference in theshape of the meteor velocity distribution for the different pointing directions. Preliminaryanalysis of these distributions indicates that we detect at least four particle populationscharacterized by their geocentric velocities: A slow one with a classical value ( $15 km/sec), an intermediate velocity population ($30 km/sec) the presence of which depends onecliptic latitude and longitude, and two fast and dominant (at sunrise) populations ($45km/sec and $50 km/sec). Finally, we explore the possibility of observational biases in ourtechnique and find no evidence for large effects. INDEX TERMS: 2129 Interplanetary Physics:Interplanetary dust; 2499 Ionosphere: General or miscellaneous; 6015 Planetology: Comets and Small Bodies:

Dust; 6245 Planetology: Solar System Objects: Meteors; 6952 Radio Science: Radar atmospheric physics;

KEYWORDS: radar meteors, meteoroids, geocentric velocities, ionosphere

Citation: Janches, D., M. C. Nolan, D. D. Meisel, J. D. Mathews, Q. H. Zhou, and D. E. Moser, On the geocentric micrometeor

velocity distribution, J. Geophys. Res., 108(A6), 1222, doi:10.1029/2002JA009789, 2003.

1. Introduction

[2] The introduction of High Power and Large Aperture(HPLA) radars as instruments to detect and study micro-meteors have raised controversies regarding the geocentricvelocity distribution of dust size particles. Observationsperformed at various HPLA facilities around the worldhave resulted in faster distributions [Sato et al., 2000; Huntet al., 2001; Janches et al., 2001] than the generally found

with classical radars [Verniani, 1973; Taylor, 1995]. Pre-vious results obtained using the 430 MHz Arecibo Observ-atory (AO) radar in Puerto Rico show a slow component ofthe micrometeor geocentric velocity distribution with a peak

$20 km/sec and a faster and dominant component with apeak$50 km/sec [Janches et al., 2001]. The majority of theobservations reported using HPLA radars were performedduring meteor showers, most of them covering the Leonidsperiod, and this could be used as an argument towards theexplanation of the high velocities. However, every previouswork concludes, based on orbital studies and meteor rates,that HPLA radars are not sensitive to meteor showers andthat the detections are mainly due to sporadic meteor

activity [Brown et al., 2001; Erickson et al., 2001; Huntet al., 2001; Janches et al., 2001, 2002; Mathews et al.,2001; Pellinen-Wannberg et al., 1998]. This result is notsurprising since IRAS studies of comet trails suggestparticle sizes greater than 1 mm [Sykes et al., 1990; Brown,1999] and most of the reported sizes derived from HPLAmeasurements are smaller.

[3] Although meteor showers are spectacular to see, themeteoric mass influx into the Earths upper atmosphere inthe submilligram mass range is dominated by the sporadicbackground. Zhou et al. [1999] argue that 60% of the totalionization in the E-region of the ionosphere is produced bydirect meteoric deposition. On average, meteor showers

enhance the night time electron concentration [Lovell,1954; Dressler and Murad, 2000]. However, not all the

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. A6, 1222, doi:10.1029/2002JA009789, 2003

1Arecibo Observatory, Arecibo, Puerto Rico.2Communication and Space Sciences Lab, Penn State University,

University Park, Pennsylvania, USA.3Department of Physics and Astronomy, SUNY-Geneseo, Geneseo,

New York, USA.4Department of Manufacturing and Mechanical Engineering, School of

Engineering and Applied Science, Miami University, Oxford, Ohio, USA.5University of Illinois at Urbana-Champaign, Department of Physics,

Urbana, Illinois, USA.

Copyright 2003 by the American Geophysical Union.0148-0227/03/2002JA009789$09.00

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showers appear to have this effect and the night timeelectron concentration during shower periods have largervariabilities than during non-shower periods [Zhou et al.,1999].

[4] The implications of a fast micrometeor geocentricvelocity distribution cover a wide range of issues. Olsson-Steel and Elford [1987], using 2 MHz observations of

meteor trails suggested that the deficiency of observedparticles in the range 106102 g [Hughes, 1978, Figure17] may be due to the presence of a faint and high velocitycomponent that has been undetected by classical meteorradars (see section 4). Mathews et al. [2001] reportedwhole-Earth yearly micrometeoroid mass flux utilizingAO radar micrometeor observations. The fluxes reportedin that work are of the order of 12 106 kg/year, 20 timeslower than the widely used flux derived from the analysis ofthe Long Duration Exposure Facility (LDEF) data [Loveand Brownlee, 1993]. To address this difference, Mathewset al. [2001] revised the LDEF results using meteor averagevelocities of $4550 km/sec instead of the 16.5 km/sec

used by Love and Brownlee [1993]. The flux resulting fromthis reexamination agreed well with the ones reported by

Mathews et al. [2001] and those by Ceplecha et al. [1998],who used satellite results to estimate the mass flux in themeteoroid mass range discussed in this paper. A similarreexamination of the LDEF results was previously reportedby Taylor [1995] using the reappraised velocity distribu-tions from the Harvard Radio Meteor Project which resultedin an average meteor velocity of 23.6 km/sec. That authorconcluded that the fluxes were 5 times lower than the LDEFresults for particles with average diameter of 175 mm. Bothreexaminations illustrate the strong dependence of meteorgeocentric velocities on meteoric mass flux estimates,specially at the smaller size particles.

[5] The precise determination of geocentric meteor veloc-ities is crucial for modeling ablation, Murad [2001] arguesthat high velocities imply that heating processes and mete-oric mass evaporation would start at higher altitudes thancurrent models predict. Most of the theoretical models thatcalculate the ablation of meteors upon atmospheric entryuse both the LDEF flux results and low average velocities[Frandorf, 1980; Flynn, 1989; Love and Brownlee, 1991,1994; Rizk et al., 1991; Farley et al., 1997; McNeil et al.,1998; Flynn, 1999; Kalashnikova et al., 2000] and areexamination of these models using higher velocities (withcorresponding lower meteor fluxes) is necessary.

[6] From an astronomical point of view, high velocities

matter. Divine [1993] proposed a model in which theinterplanetary meteoroid environment is composed of fivedifferent meteoroid populations, each one with differentparticle mass and orbital parameters. Orbital properties aredirectly dependent on particle velocity and it is unknownhow much fast micrometeor velocity distributions wouldinfluence the existing models. In addition, HPLA radarsdetect smaller particles than the radar meteor observationsutilized in Divines model. Micron-sized particle data werepreviously provided by zodiacal light measurements andsatellite dust detectors [Grun et al., 1985, 1992; Love and

Brownlee, 1993; Ceplecha et al., 1998; Grun et al., 2001,and reference therein] which generally do not give particlevelocities and radiants. Dikarev et al. [2002] recentlyreported an improved model of the interplanetary dust

environment utilizing more recent data and outlined theimportance of introducing HPLA radar meteor results to thecurrent modeling efforts. In addition, self-consistent modelsof asteroid collisional evolution that examine a collisionalcascade, where asteroids are broken into pieces by collisionswith (more numerous) smaller particles, increasing thepopulation of small particles, need to assume size- and

velocity-distribution of dust [e.g., Campo Bagatin et al.,1994]. Finally, current models of the Solar System dustbands [Dermott et al., 2001; Grogan et al., 1997, 2001] useLDEF dust fluxes as well as classical average particlevelocities and it is unclear how much influence lowermeteor fluxes and higher velocities would affect theirresults.

[7] We present initial results from an ongoing program ofobservations and study of the micrometeor flux into theupper atmosphere that began in January 2002 using the dual-beam 430 MHz Arecibo Observatory (AO) radar. The use ofdual radar beams lets us observe two separate regions of thesky at the same time. The observations performed in January

and June 2002 have been analyzed and are discussed in thispaper. The data from both months clearly show that themicrometeor velocity distribution is dependent on topocen-tric declination, suggesting structure in the geocentric micro-meteor radiants. We describe our observation technique insection 2 and present and discuss our results in section 3.Observational and instrumental biases in our technique arediscussed in section 4. The issue of bias needs to be resolvedto determine whether the velocity distributions observed byAO, and HPLA radars in general, represent absolute distri-butions. We also outline in section 4 the principal differencesbetween classical meteor and HPLA radars. A summary ofthe work reported here and discussion on future research ispresented in section 5.

2. Observing Methodology

[8] The National Astronomy and Ionosphere Center(NAIC) Arecibo Observatory (AO) has a 305-m diameterspherical reflector antenna and two feeds that can operatesimultaneously at 430 MHz. The AO system is used forradio astronomy studies, planetary radar investigation, andionosphere research. Incident parallel electromagneticwaves reflecting from a spherical mirror will focus on aline; hence the original feed is a line feed (LF). A LF has arelatively narrow frequency band width which has limita-tions for radio astronomy studies. For more bandwidth the

second feed is a dual-reflector or Gregorian (GF) feed thatincludes two subreflectors and a feed horn. With both theLF and the GF in place we can divide the transmitted powerin two and achieve a dual-beam capability to the 430 MHzAO radar. Thus AO transmits two separate 430 MHz radarbeams simultaneously, probing two separate regions of theEarths ionosphere at the same time. In our experiment theAO radar detects over 10,000 meteor events daily insidethe 300-m-diameter radar beam. Each month, we collect aminimum of one 14 hour interval of observations (18000800 AST). This interval is chosen to cover the peak of thediurnal sporadic micrometeor flux that is detected daily byAO (or any HPLA radar) [Janches et al., 2002; Mathews etal., 2001; Pellinen-Wannberg et al., 1998]. The LF antennawas pointed vertically during the entire observing period

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while the GF was pointed 15 degrees from the zenith. Theoff-vertical radar beam was initially placed pointing northand every 30 minutes was rotated 180 degrees. Thismethodology allows us to monitor the meteoric flux rightabove AO (astronomical coordinates, 1821013.700 northlatitude, 6645018.800 west longitude) and 15 degrees southand north declination from the zenith.

[9] Figure 1 shows the event rates detected by AO at thethree different directions during the observations performedon the local night/morning of 21 22 January and localmorning of 25 June 2002. No great dependence of meteorrate with geocentric declination is evident in these figures.There is, however, a strong modulation of the event occur-rence rate for the meteors observed with the GF pointingnorth from AO after 0300 AST. The fact that this modu-lation is only seen when we point in the North directionwith the GF suggests that this is not an instrumental effect,otherwise the same modulation should exist for the Southpointing results. Interestingly, in June the same modulationis present also in the North pointing direction and perhaps

with smaller amplitude. This is the first indication that weare observing different populations of particles in the near-Earth dust environment. Further discussion on this issue ispresented in the next section when we introduce ourvelocity results.

[10] The AO beam covers a relatively small (comparedwith other radars) portion of the sky ($0.07 km2) and asit will be demonstrated later, the meteor traveling directionis parallel to the beam axis. This implies that the ratesdisplayed in Figure 1 represent the meteor flux comingfrom a given direction in the sky (radiant) rather thanthe flux resulting from a wide angular radiant distribution.The high event rate early in the local morning is due tothe fact the AO radar points towards the so-called apex(the direction of the Earths motion in its orbit around theSun) and particles caught by the Earth, as well as head-oncollisions, are present, unlike the antapex direction wherea particle must travel sufficiently fast to reach the Earthfrom behind in its orbit [Janches et al., 2000a]. Theseobservations cover the 80 140 km altitude range with150-m height resolution. Figure 2 displays the initialobserved altitude of the events detected by the AO radarin the three different observed directions. The radarvisibility distribution peaks at $107 km, agreeing withother HPLA radar observations [Hunt et al., 2001; Erick-

son et al., 2001], illustrating also the fact that HPLAradars do not have the strong ceiling effect that conven-

tional meteor radar have [Olsson-Steel and Elford, 1987;Steel and Elford, 1991].

[11] Our pulse scheme has evolved considerably fromour previous reports [Janches et al., 2000a, 2000b, 2001]and is illustrated in Figure 3. Figure 3a shows the range-time-intensity (RTI) plot showing a meteor event that isobserved first at $107 km and shows a vertical rangeextent that corresponds to the 45 msec uncoded radar pulse;the interpulse period (IPP) is 1 msec. We sample the radarreturn every 1 msec resulting in the 150-m height resolu-tion. The vertical pattern in the noise background of theRTI plot is due to elemental incoherent scattering fromthe ionosphere. Figure 3b displays the signal-to-noise-ratio (SNR) and velocity plot for the event presented inFigure 3a. The data points are the instantaneous Doppler

velocity determinations. The RMS fitting errors, smallerthan the symbols, are a few m/sec for an event with initialspeed of $45 km/sec. A least-square fit to the velocityresults in a constant deceleration of 21.4 km/sec2 for thisparticular case. Constant decelerations are consistentlymeasured in the AO radar meteor events [Janches et al.,2000b]. In general, using this Doppler technique, instanta-

neous meteor radial velocities are obtained with errors of theorder of 10100 m/sec. Tangential velocities are not mea-sured. Meteors with large tangential velocities are statisti-cally eliminated from the sample prior to orbit determinationusing a consistency check based on upward integrationthrough the atmosphere [Meisel et al., 2002]. With precisemeteor altitudes, radial velocities and radial decelerations,and a model atmosphere, meteoroid mass can be estimatedusing the meteor momentum equation [Janches et al.,2000b], resulting in a meteoroid size range detected byAO of 0.5 mm100 mm in radius [Mathews et al., 2001].

3. Results

[12] The meteor topocentric radial velocity distributionsfrom the January observations are displayed in Figure 4.Each panel in Figure 4 represents an interval of 2 hours. Thetop panels are the velocities of the events detected by theGF when looking north of AO (zenith distance = 15,azimuth = 0), the middle panels are those events detectedby the LF (zenith distance = 0, azimuth = 0), and thebottom panels are the events detected by the GF when it waspointed south of AO (zenith distance = 15, azimuth =180). All distributions are normalized for clarity. Thepanels in Figure 4a display the velocity distributions ofthe events detected during the night of 21 January whileFigure 4b displays the distributions from the eventsobserved during the morning of 22 January. The event rateduring the evening is low (Figure 1) and the velocities arelower because the meteors are produced mainly by meteo-roids that reach the Earth from behind [Janches et al,2000a]. It can be seen from these plots that starting aftermidnight a low velocity background (peak $35 km/sec) ispresent early in the evening until approximately 0400 AST,when a high velocity component (peak $50 km/sec) dom-inates the distributions. The average speeds over the wholeperiod of observation are 31/38/38 km/sec for the North/Vertical/South pointing directions. These average velocitiesare somewhat lower than those reported by Mathews et al.

[2001] (45 50 km/sec) using similar observations per-formed in November 1997/1998. However, they are largerthan the accepted average sporadic meteor speed derivedusing classical radars or optical observations (16.9 km/sec,

Erickson [1968]; 23 km/sec, Taylor[1995]) and of the sameorder of the radar meteor velocities reported by Verniani[1973] (34 km/sec) and Jones and Brown [1993].

[13] A very surprising result is the dependence of thedistribution shape with topocentric declination, whichimplies a function of radiant ecliptic latitude. Starting at0200 AST the distributions show a noticeable difference inshape. From the three panels corresponding to the 04000600 AST interval it can be observed that, while the Northdistribution has a single peak (

$42 km/sec) and lower

average velocity, the South distribution is clearly bimodal

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Figure 1. Observed event rates by the 430 MHz AO dual-beam radar system on the local evening/morning of 2122 January and local morning of 25 June 2002. (a) Events detected by the Gregorian feed(GF) when looking North from AO. (b) Event rate detected by the line feed (LF) pointing verticallyabove AO. (c) Meteor rate detected by the AO-GF pointing South from AO. The intervals with no data inFigures 1a and 1b represent the time that the GF was pointing in the opposite direction plus the time thattakes the azimuth to rotate 180 degrees. (d, e, and f) The same as Figures 1a, 1b, and 1c but for the June

observations.


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(slow peak at$15 km/s, fast peak at$47 km/sec) with veryfew events with velocities of the order of 30 km/sec. TheVertical distribution seems to be a mixture of the Northand South, where the bimodal characteristics are stillpresent but with some 30 km/sec objects. The same charac-

teristics are present during the 0600 0800 AST period.Note that the lack of events with velocities of the order of30 km/sec cannot be due to an instrumental bias because weare using the same instrument to monitor two different areasof the sky. If the absence of meteors were due to aninstrumental bias, then the effect should be present in boththe North and South distributions.

[14] In order to verify and explore the causes of thesefeatures we analyzed the June observations, leaving the dataacquired during the months in between for later analysis. Webegan the observations at 0230 AST in the morning of 25June 2002 and we observed until 0800 AST. The detectedfluxes during these observations are similar to those detected

in January ($35 events per minute at 0500 AST). Figure 5shows the resulting normalized velocity distributions from

the June observations. The scenario is similar from that inJanuary. Like the distributions derived from the Januarydata, the bimodal shape is present in the South distribu-tion. This distribution again shows a lack of objects withatmospheric entry velocity around 30 km/sec. In theNorth direction the velocity distribution is slower andwith a single peak, while the Vertical distribution again

seems to be mixture of the off-vertical results. In addition, allthe distributions seem to be faster than the ones observed inJanuary. The average meteor velocities over the entire periodof observation are 39/42/42 km/sec for the North/Vertical/South radar pointing directions. The dashed lines in the

Figure 3. (a) Range-time-intensity (RTI) plot showing atypical meteor event that begins at $107 km and shows arange extent that corresponds to the 45 msec uncoded radarpulse. The interpulse period (IPP) is 1 msec and the samplerate is 1 msec (150 m). The pattern in the noise backgroundof the RTI plot is due to elemental incoherent scatteringfrom the ionosphere. (b) Signal-to-Noise-Ratio (SNR) andVelocity plot. The data points are the instantaneous Dopplervelocity determinations. The RMS fit errors are a few m/secfor an event with initial speed of $45 km/sec. A least-

square fit to the velocity results in a constant deceleration of21.4 km/sec2.

Figure 2. Observed meteor initial altitudes for the threepointing directions. Note this figures shows the AOvisibility distribution which peaks at $107 km.


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panels corresponding to the later intervals are fitted Gaussianfunctions parameterized as

f V A0 exp V V

2

s2

!1

where V is the mean velocity and s is the standarddeviation. The fitted parameters for the different datasets aregiven in Table 1. The results presented in Table 1 confirmthat the June observations resulted in faster distributions.The complete year-long data set analysis will distinguishwhether if this difference is due to a seasonal variation ofthe atmospheric entry micrometeor velocity, orbital struc-

tures of the dust environment at 1 AU, or short-term effectsin the dynamical characteristics of micrometeors. Note that

in all the latest (04000800 AST) intervals a very narrow($2 km/sec) distribution with a peak at $50 km/sec ispresent. It is not clear yet if this is statistical fluctuation or areal population, but the count rate is clearly over the noiselevel. In summary, preliminary analysis of these distribu-tions indicate that we may be observing at least four particlepopulations characterized by their geocentric velocities: aslow one with a classical value ($15 km/sec), anintermediate velocity population ($30 km/sec) that is notalways present, and two fast and dominant (at sunrise)populations ($45 km/sec and $50 km/sec).

[15] Figure 6 displays the AO radar beam pointinginformation during both observations. In this figure the

ecliptic latitude (b) is shown as a function of time. Therange of ecliptic latitudes covered during the different

Figure 4. Observed geocentric meteor velocity distributions on the local evening of 21 January 2002 (a)and morning of 22 January 2002 (b). Each panel represents an intervals of 2 hours (columns) for the threedifferent areas of the sky under study (rows). These plots show the presence of a low velocity background(peak $30 km/sec) early in the evening until approximately 0400 AST, when a high velocity component(peak $50 km/sec) dominates the distribution. Note that starting at 0400 AST the distributions show anoticeable difference in shape. From the three panels corresponding to the 04000600 AST it can beobserved that while the North distribution has a single peak ($42 km/sec) and lower average velocity,the South distributions is clearly bimodal (slow peak at $15 km/s, fast peak at $47 km/sec) with afaster average velocity. The dependence of these features with time of year will be studied as moreobservations are analyzed.


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intervals are also presented in Figures 4 and 5. At midnightin January the sky accessible to the GF pointing south ofAO and the LF pointing vertical is below the ecliptic planewhile for the GF pointing North is just above the eclipticplane. On the other hand, for the same time in June, thethree different pointing directions are all well above theecliptic plane. However, as shown in Figure 1, the large partof the meteor flux occurs after 0200 AST and it is presentuntil the end of the observations at 0800 AST. During thisperiod the radar beam in the three directions is at the sameecliptic elevation for the two months discussed here.

[16] From an instrumentation point of view the differencein shape of the micrometeor geocentric velocity distribu-tions show that the meteoroid entry directions are mainlydown the beam (within 15 degrees), as we have previouslyassumed [Janches et al., 2001; Meisel et al., 2002], solvinga long standing dispute. We refer to the trajectories asdown the beam because the radar is strongly biased toobserve meteors near (within 1520 degrees) the axis of theradar beam. If meteors were to travel across the beam inrandom directions, there is no reason why the distributionshould look significantly different when looking in slightlydifferent directions. A degree of uncertainty in previouslydetermined AO radiants of about 15 degrees (the same as

the angle between radar beams used here) was suggested byMeisel et al. [2002] using dynamical arguments. Ongoing

observations using smaller zenith distance separations mayreduce this uncertainty even more. In addition, preliminarycalculations using the meteor energy equation [Whipple,1950; Bronshten, 1983; Ceplecha et al., 1998] indicate thatif the micrometeor atmospheric entry angle is greater than30 degrees with respect to the vertical direction, ablationwill occur at greater altitudes than those observed (Figure 2).Thus AO is sensitive to down-the-beam travelling objectsbecause meteors with large entry angle would ablate beforereaching the 100 km altitude region and is not an effectproduced by the radar scattering mechanisms of the meteor

head-echo (see next section). The fact that two beams only15 degrees apart reproducibly see different results showsthat the sensitivity to across the beam objects is low. Recentmeteor measurements reported using the Advance ResearchProjects Agency Long-Range Tracking and InstrumentationRadar (ALTAIR; Marshall Islands) [Brown et al., 2001;Close et al, 2002] show a broader set of angles with respectof the radar beam. However, ALTAIR has a large beam-width relative to AO and thus does see highly inclinedmeteors. We see them but they must be short events and thedynamic criteria are not satisfied. This implies that statisti-cally speaking, these distributions represent radiant velocitydistributions and not just radial velocity components, espe-

cially for high geocentric velocities. The low-velocity sideof the distribution may indeed contain across the beam

Figure 4. (continued)


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meteors, as the statistical criteria for eliminating them havenot been applied to these distributions. In any event allmeteors with radial velocities less than the geocentrichyperbolic limit are not used for orbital analysis [Jancheset al., 2001; Meisel et al., 2002]. Furthermore, the fact thatwe measured the same behavior in the velocity distributionswhen looking at the same ecliptic latitude but approximately

Figure 5. Observed geocentric meteor velocity distributions on the local morning of 25 June 2002, 5months after those of Figure 4. The arrangement of the panels is the same as Figure 4.

Table 1. Geocentric Micrometeor Velocity Distribution Gaussian

Fitted Parameters

PointingDirection Month

TimeInterval, LT

A0 V, km/sec s, km/sec

Fast Slow Fast Slow Fast Slow

North January 0400 0600 0.8 38.1 3.3Vertical January 0400 0600 0.9 0.2 46.0 25.6 2.8 3.9South January 0 400 0600 0.8 0.2 47.6 15.5 2.5 2.6North January 0600 0800 0.8 38.1 3.6Vertical January 0600 0800 0.7 0.1 48.9 34.9 2.8 4.6South January 0 600 0800 0.9 0.2 50.1 18.9 2.7 3.0North June 0400 0600 0.6 43.6 3.5Vertical June 0400 0600 0.8 0.2 48.0 26.1 3.0 4.1South June 0400 0600 0.7 0.1 50.8 15.7 2.4 2.7North June 0600 0800 0.7 46.0 3.0

Vertical June 0600 0800 0.9 0.1 50.4 18.0 2.4 3.3South June 0600 0800 0.5 0.1 52.5 16.0 4.0 2.7 Figure 6. AO pointing information for the observationsperformed in January and June 2002.


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180 apart in ecliptic longitude suggests that the depend-ency of the shape of the velocity distributions with eclipticlatitude and longitude is a real feature of the interplanetarydust environment at 1 AU.

4. Differences and Similarities Between Classical

Meteor and HPLA Radars: Is There a Biason the AO Velocity Measurements?

[17] Before the use of HPLA radars, dust-size particledetections were done either with spacecraft detectors, zodia-cal light measurements, or with low power HF/VHF classicalmeteor radars. The first two techniques do not provide directparticle velocity measurements and when needed they usevelocity distributions derived from classical meteor radars.Classical meteor radars base their velocity results on thedetection of the Fresnel diffraction patterns from the devel-oping trail [McKinley, 1961; Ceplecha et al., 1998, andreference therein]. The use of the developing trail as amethod for determining meteor speed has several detection

biases that need to be corrected for when reporting meteoraltitude and velocity distributions. These biases are presentdue to various radar sensitivity effects inherent in thisapproach. The first is the trail orientation effect, where theFresnel diffraction intensity is very sensitive to angulardeparture from the meteor perpendicular path with respectthe radar beam axis. The second effect is the height ceilingeffect [Olsson-Steel and Elford, 1987; Steel and Elford,1991]. Classical trail radars do not see the highest geocentricvelocities very well because of the tendency of fast meteorsto form trails higher in the atmosphere where the diffusiontimes are very short. Those trails simply disappear veryrapidly before they form over a Fresnel zone (12 km) sothat, even though they can provide strong echoes, velocitycannot be determined from them. Steel and Elford [1991]have suggested that 50% of all meteor trains are undetectedabove 105 km. Note that this is the altitude at which thevisibility rate peaks for AO (Figure 2) and other HPLAradars [Hunt et al., 2001; Erickson et al., 2001] and extendswell above this altitude. Trail diffusion effects can becompensated for by going to ultra low frequencies. Olsson-Steel and Elford [1987] reported meteor observations at2 MHz, wavelength for which the trail echo ceiling occursat 140 km. The resultant height distribution is found to be at$104 km, 10 km higher than the peak detected by classicalradars and in agreement with the distributions measured byHPLA radars. On the other hand, for slow meteors the

collisional ionization probability decreases [Bronshten,1983] presenting difficulties of detection at the lower endof the speed distribution. Jones [1997] points out that thiseffect is particularly strong for slow meteors (V 35 km/sec)and that ionization coefficients for these velocities is $V3

dependent. Finally, there is a mass bias in classical trailformation. The larger the meteor mass, the lower in theatmosphere the trails occur; thus there is a distinct mass-velocity correlation that must be disentangled for trails. It hasbeen argued [Murad, 2001] that such biases may exist in themeteor observations using HPLA radars, and we dedicatethis section to a preliminary exploration of this issue.

[18] As it has been discussed in a previous section, wemeasure the instantaneous velocity using a Doppler techni-que from the consistent detection of the meteor head-echo.

If the scattered signal from the meteor head echo is strongenough to be detected by AO, then we can determine itsvelocity. Note that this characteristic is completely differentfrom the case of classical radar. In that case the returnedsignal from a meteor trail can be detected by the radar, but ifthe meteor trail did not form over an entire Fresnel length,then the meteor velocity will not be determined. This kind

of limitation is not present using our technique. The ques-tion remaining then is: does the formation of the head-echodepend on meteor parameters (i.e., altitude, velocity, etc)?.

[19] The origin and nature of the head echo are not wellunderstood yet and many efforts trying to explain it usingHPLA radars have been reported. Wannberg et al. [1996]measured radar meteor head-echoes using the VHF/UHF(224/930 MHz) European Incoherent Scatter (EISCAT)radar systems in northern Scandinavia and argue thatsingle-particle overdense echo scattering is sufficient toexplain their observations. Mathews et al. [1997] use earlymeteor observations with AO to argue that the returnedradar signal is from coherent scattering from a small

(compared with the radar wavelength) ensemble of electronssurrounding the meteoroid. Close et al. [2000, 2002]reported for the first time high polarization ratio measure-ments of meteor head-echoes utilizing ALTAIR, suggestingthat the meteor head-echo is approximately spherical inshape. The authors agree with Wannberg et al. [1996] thatthe head-echo scattering results from an overdense plasma.J. D. Mathews et al. (An update on UHF radar meteorobservations at arecibo observatory, submitted to Journal of

Atmospheric and Solar-Terrestrial Physics, 2003, herein-after referred to as Mathews et al., submitted manuscript,2003) give a detailed summary of these issues.

[20] Regardless of the mechanism that produces themeteor head-echo, once the radar scattering cross-section(RCS) is sufficiently large to produced a scattered signalstrong enough for the radar to detect it, then the velocity canbe estimated using Doppler techniques. Since the ionizationmechanism responsible for the observed head echoes is notfully understood, we must consider the effect of observa-tional biases in our results, particularly as a function ofvelocity. As discussed earlier in this section, such biases arevery strong in measurements of classical meteors radars. Weassume that there is a bias and then show that it cannot be apower-law function of velocity as strong as that seen inconventional meteors radars [Galligan and Baggaley,2001]. The possibility of a threshold bias remains. Figure7 shows a cartoon of this problem. We first assume that for a

given mass, the relation between the RCS of the head-echoand the meteor velocity follows a power law (Figure 7a). Inthat case, the logarithm of sRadar versus the logarithm ofmeteor velocity plot should have a triangle-like distribution,where the base of the triangle is determined by the mini-mum sRadar that AO can detect for the given mass range.The vertical spread (altitude of the triangle) is produced bythe position of the meteoroid within the radar beam (a largemeteoroid close to the edge of the beam will appear to havea lowersRadar than a smaller particle traveling through thecenter of the beam) and the mass range under study. Theslope of the triangle is determined by the power law.Perhaps there are many meteors with cross sections belowthe sensitivity limit, and a lower limit would showthe dependence. We cannot easily lower the detectability


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threshold as we are already using the most sensitive radar inthe world, but we can raise the cross sections. The detectionof larger masses (and thus we assume larger cross sections)should compensate in some manner for the velocity bias

allowing the AO radar to observe more low velocity objects(Figure 7b). The percentage of detected slower particlesshould thus increase as the mass-range increase, since thelarger cross section should compensate for the bias. On theother hand, if there is no strong velocity bias, detectinglarger masses should simply increase the detected RCSindependently of meteor velocity (Figures 7c and 7d).Mathews et al. (submitted manuscript, 2003) argues thatthe instantaneous head-echo scattering cross-section islikely proportional to the time rate of energy deposition(i.e., meteor energy mixed with atmospheric mass density)but with some modifications. Especially at 430 MHz, theplasma is optically thin and thus, if less than l/4 in

longest dimension, the RCS is proportional to N2

. Aselectrons spill out of this dimensional constraint the cross

section no longer grows. On the whole, a larger massmeteoroid at constant altitude and constant velocity willproduce a larger RCS.

[21] The meteor RCS (sRadar) is defined by the radar

equation, assuming the meteor to be a point target [Mathewset al., 1997; Close et al., 2002], as

sRadar

Pn 4p 3 h

cos ZD

4PTl

2G2SNR 2

where PT is the radar transmitted power ($0.5 MW for theJanuary observations), l is the radar wavelength (69.7 cm),G is the antenna gain ($60 dBi at 100 km), h is theobserved meteor altitude, ZD is the zenith distance, and Pnis the noise power given by

Pn kTsysB 3

Figure 7. Cartoon exploring two observational biases. (a and b) A power law dependence of the meteorhead echo with meteor velocity. (c and d) A nonbiased distribution. See section 4 in the text for details.


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where kis the Boltzmann constant, B is the noise bandwidth(1 MHz), and Tsys is the system temperature ($120 K). Notethat for this analysis we are utilizing only the LF detections( Z D = 0), although the GF detections show the sameresults.

[22] Figure 8 displays sRadar as a function of meteorvelocity divided in mass ranges as determined by the AOdata. The transmitted power is measured with 10% uncer-tainty and the system temperature can vary between 10 to20% during the night; hence the errors on the sRadar areabout 30% completely negligible compared with the factthat sRadar covers 3 orders of magnitude for a given massrange. As seen in Figure 8, while the vertical spread andthe horizontal limit are present, there is no suggestion thatany strong dependence of sRadar with meteor velocityexists, implying that the RCS of meteor in a given massrange is largely independent of velocity. It is clear fromthis figure that the sensitivity of the experiment for this

particular set of observations is approximately sRadar $109m2. The RCS of the larger particles should be larger,

and we see that in the large mass bins the typical crosssection in the higher velocity ranges has gone up by afactor of $10. The cross sections of the lower-velocitymeteors has also gone up, but the fraction of low-velocitymeteors has not appreciably changed. In all the panels of

Figure 8, the percentage of particles with velocity lowerthan 30 km/sec is $20%. Thus there is no hidden pop-ulation of meteors just below the detectability threshold,and the velocity bias is either quite small or else extremelystrong (a threshold bias). If the velocity bias is a thresholdbias, then perhaps the observed low-velocity meteors onlyhave low apparent velocities due to geometric effects(events with large tangential velocities). However, Figures4 and 5 shows that the velocity distribution has both a low-and high-velocity component for some observations andthe presence of meteors with intermediate velocities ($30km/sec) depends on radiants. This effect cannot be pro-duced by an instrumental bias, which would be independ-

ent of pointing. We thus reject the hypothesis of purelygeometric effects.

Figure 8. Radar scattering cross section versus meteor velocity plots arranged in factor of 10 mass-range intervals. The last panel contains 3 orders of magnitude mass interval.


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[23] Finally, a study can be done in order to determine ifthere is any dependence between radar detectability of themeteor head-echo and initial observed altitude. Figure 9ashows the distribution of the initial observed altitude sep-arated in three velocity ranges. As previously discussed, the

traveling direction of the detected particles is assumed to beparallel to the radar beam; hence these altitudes representthe height at which the head-echo have sufficient amount ofelectrons to be initially detected by the AO radar. Asdiscussed in the previous section, these altitudes also agreewith ongoing theoretical calculations of the initial ablationaltitude using the meteor energy equation approximated to

small particles. Note that independent of the velocity range,the peaks of each distribution occur at altitudes close to thepeak of maximum altitude radar visibility (107 km; seeFigure 2). Figure 9b shows the average initial altitude as afunction of velocity for velocity bins of 5 km/sec. We seethat a dependence of velocity with visibility height ispresent. However, all the average initial altitudes are insidethe altitude range for which radar visibility is maximum.These results indicate that the meteor head-echoes detectedby AO do not have a strong fast dissipation bias likehigh-altitude meteor trails do. We conclude that the datashow no evidence that a strong velocity or altitude depend-ent bias exists that will prevent both fast and slow particles

from being detected by AO and consequently modifyentirely the radial velocity distributions detected at AO.The discussion presented in this section is summarized inTable 2 which represents an updated version of Table 1given by Pellinen-Wannberg [2001].

5. Summary

[24] We report micrometeor geocentric velocity distribu-tions obtained using the 430 MHz Arecibo radar and its newdual-beam capabilities. The AO line feed is pointed at thelocal zenith and the Gregorian feed at a zenith distance of 15degrees looking North or South. By rotating the azimuth

180 degrees every 30 minutes, the simultaneous study of themicrometeor flux right above AO and North and South fromAO is achieved. The observations presented here are part ofa year-long campaign that began in January 2002 to monitorthe micrometeor flux into the Earths atmosphere with thehighest velocity and radiant resolution reported so far.Results from the observations performed on 21 Januaryand 25 June are presented and discussed. The AO radardetects over 10,000 meteor events daily inside the 300-mradar beam, mainly produced by particles in the size-range0.5 100 microns. It is important to note the particlepopulation detected by AO is smaller in particle size than

Figure 9. (a) Observed initial altitude distribution for threemeteor velocity groups. (b) Average observed initial altitudeversus meteor velocity.

Table 2. Similarities and Differences Between Conventional and HPLA Radarsa

Conventional Radars HPLA Radars AO Radar

Power $W to kW MW 1 MWBeam Width !3.2 1 1

6

Typical Frequency 50 MHz MHz !40 MHz 430 MHzAntenna Type Yagis Reflectors or Phased Arrays Spherical reflector Echo type Trail echoes (mainly) Head echoes (mainly) Head echoesScattering Orientation Beam-perpendicular Isotropic IsotropicVelocity Determination Fresnel diffraction Time-of-flight or Doppler Time-of-flight or Doppler Meteor origin Showers/Sporadic Sporadic SporadicIonization bias $V3 Not known yetb not known yetHeight ceiling effect Present Not Present Not Present Size-range detected !40 mm claim

based on flux500 mm based

on decelerations fluxes and RCS0.5100 mm based

on decelerations fluxes and RCSaThe second column represents a summary of many facilities around the world. We mainly refer to those radars used for Incoherent

Scatter studies of the Earths ionosphere.bUsers of the different HPLA radars facilities may disagree on this point.


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the population observe using HF/VHF classical meteorradars. The micrometeor instantaneous atmospheric entryvelocities are obtained using a Doppler technique thatprovides speeds with errors of the order of 10 100 m/secfor all meteors detected. The average velocities seem to bein agreement with other studies, most of which also useHPLA radars [Verniani, 1973; Sato et al., 2000; Hunt et al.,

2001; Janches et al., 2001; Mathews et al., 2001]. Howeverthey seem to be faster than the standard belief [Erickson,1968; Verniani, 1973; Taylor, 1995]. A surprising result isthat the shape of the meteor velocity distribution seems todepend on ecliptic coordinates. In January a clear bimodaldistribution was measured for the observations closer to theecliptic, while a slower and single-peak distribution wasobserved further away from the ecliptic plane. The situationrepeats 6 months later, as the June observations have shown.A similar asymmetry is observed in the meteor event rate. Astrong modulation on the meteor flux was observed in theNorth pointing direction during both months of observa-tions. Preliminary analysis of these distributions indicate

that we detect at least four particle populations characterizedby their geocentric velocities: a slow one with a classicalvalue ($15 km/sec), an intermediate velocity population($30 km/sec) the presence of which depends on eclipticlatitude and longitude, and two fast and dominant (at sun-rise) populations ($45 km/sec and $50 km/sec). The 50km/sec population seem to have a very narrow speeddistribution, and its count rate is well above the noise level.The details of how these differences were produced are notyet understood. There are a number of ways to produce thevelocity distributions observed including electromagneticeffects [Morfill and Grun, 1979] or orbital structures in thezodiacal dust (IRAS bands; Low et al. [1984]). High meteorvelocities have been observed before [Jones and Brown,1993; Taylor et al., 1994, 1996; Galligan and Baggaley,2001], even though classical meteor radars are not veryefficient at detecting high-speed meteors because they areproduced at higher altitude where the meteor trail diffusevery rapidly (see section 4). These meteors were a small partof the total distribution, and were presumed to be either(rare) interstellar dust or to be the high-velocity tail of aMaxwellian distribution. In this study, where we observedmuch smaller particles than previous studies, we clearlydistinguish separate populations of high-velocity particles.We observe a spatial variability in the population of 1100micron-sized dust entering the Earths atmosphere that is notconsistent with the usual idea of comet dust mixed with

collisional debris from the asteroid belt, all drifting slowlyinward due to Poynting-Robertson drag. A number ofasteroid collisional evolution models have assumed that1100 micron-sized dust particles are created in the asteroidbelt by collisions and ejected from the Solar System due toradiation pressure (at 1-micron) or drift into the Sun due toPoynting-Robertson drag (at 100 microns) and are lost fromthe system. Thus they assume a decaying population ofmicron-sized dust on essentially circular orbits that spiralinto the Sun. This population is then replenished by colli-sions of larger particles. Our results show that while the(low-velocity) component does exist, there is an additionalhigh-velocity component of the dust, which will be muchmore erosive to larger particles (for a given impactor size)because the impact rate is proportional to velocity and the

impact destructiveness is approximately proportional toenergy. Dust does not fade gently into oblivion but has alast hurrah as it is accelerated to high velocity. If theseparticles are accelerated by electrostatic or magnetic forces,which could have been at least as important during SolarSystem accretion as they are today, they could have affectedthe accretion process in complicated ways. The details of

how these differences in the velocity distributions wereproduced are not yet clear but mechanisms that producethe velocity distributions observed have been proposed,such as electromagnetic effects on small charged particles[Morfill and Grun, 1979]. We are currently exploring thesequestions in some detail. Further characterization of thevariation in the dust velocity distribution will be necessaryto assess the importance of the high-velocity population tothe collisional cascade and to determine the size at whichdust does finally cease to have an effect. Our data sampleonly a small region of meteoroid phase-space, restricted to 1AU, and with velocities that lie along the beams of theradar. These experiments sampled only three different

velocity directions and saw clear and repeatable heteroge-neity. The distribution also varies with time of day (and thusecliptic longitude) on a comparable scale (30). Furtherexperiments to improve phase-space coverage are in pro-gress. Analysis of the observations performed during therest of the year will provide valuable information that mayelucidate the origin and nature of these structures andparticle populations. Finally, we explore the existence ofobservational biased in our technique to determine if thedistributions measured at Arecibo are bias towards high-velocity objects. We found no evidence to believe thatstrong effects exist artificially producing a lack of lowvelocity (

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